Holographic printer

ABSTRACT

The present invention provides a single method and apparatus for producing many of the most common types of hologram from digital data is disclosed. The data are generated entirely by a computer as a 3-D (animated) model or from multiple 2-D camera images taken of a real 3-D (moving) object or scene from a plurality of different camera positions. The data are digitally processed and displayed on a small high resolution spatial light modulator (SLM). A compact low energy pulsed laser, is used to record composite holograms. The present invention permits the creation of restricted or full parallax master transmission or reflection type composite holograms, known as H1 holograms, that can be copied using traditional methods. Alternatively the same invention and apparatus permits the direct writing of hologram without the need to pass through the intermediate stage of the H1 transmission hologram.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of the U.S. Patent Application havingthe Ser. No. 10/149,211 filed Sep. 30, 2002, which is a 371 ofPCT/GB/04699 filed Dec. 8,2000.

BACKGROUND OF THE INVENTION

The present invention relates to holographic printers. According to apreferred embodiment a method and apparatus for recording and printingholographic stereograms from digital data is disclosed.

For over 50 years holograms have been produced by the general techniqueof illuminating an object with coherent light and arranging that thescattered light falls onto a photosensitive recording material that isalso illuminated by a mutually coherent reference beam (see for instanceE. N. Leith et al., “Reconstructed Wavefronts and Communication Theory”,Journal of the Optical Society of America 53, 1377–81 1963). However,with such a technique one requires a physical object in order to make anholographic representation of this object and usually the size of theholographic image corresponds in a 1:1 fashion with the size of theobject holographed. For many practical applications this technique ishence unsuitable.

An alternative technique of generating and then directly writing thefundamental interference pattern that characterizes an hologram has beendiscussed and investigated (see for instance U.S. Pat. No. 4,701,006).However, even with today's computer resources, calculation of theinterference pattern by Fourier transforms remains a dauntingcomputational task for larger holograms. In addition it is still highlydifficult and costly to write such patterns once calculated, thepreferred technique being by electron beam.

Another technique for the generation of holograms that does not requirean actual object was proposed by King et al (Applied Optics, 1970). Inthis paper it was shown that holograms can be composed by opticallymultiplexing information taken from a plurality of 2-D camera views. Theimportance of this idea is that the machine that prints the finalholograms can be separate from the actual object and that theholographic image does not have to correspond in size to the originalobject. Further, it has been shown that an object is not required at allif the 2-D views are generated from raw computer data (see for exampleU.S. Pat. No. 3,843,225).

In a common embodiment of the above principle it is known to recordsequential views of an object by a camera mounted on a linear orcircular track. Each of the views is then used in an optical system thatmultiplexes the data together to form an intermediate (or H1) hologramsuch as described in U.S. Pat. No. 3,832,027. Such a hologram can thenbe converted or transferred to form a second hologram which is nowwhite-light viewable and is known as the H2 hologram. In order to effectthis the H1 hologram is illuminated by laser light in a time-reversedgeometry and the real image so produced is used as the object for the H2hologram. Upon illumination of this H2 hologram by a time-reversedreference beam a white-light viewable virtual image is reconstructed. Anefficient and practical commercial machine for converting H1 hologramsto H2 holograms is known (see M. V. Grichine, D. B. Ratcliffe, G. R.Skokov, “An Integrated Pulsed-Holography System for Mastering andTransferring onto AGFA or VR-P Emulsions” Proc. SPIE Vol. 3358, p.203–210, Sixth International Symposium on Display Holography, Tung H.Jeong; Ed.)

Holographic printing techniques which implicitly require the generationof an intermediate, or H1, hologram which is thereafter used to producea final white-light-viewable hologram are commonly referred to as“2-step” holographic printing processes. Essentially all the majorfeatures of known “2-step” holographic printing processes are explainedin U.S. Pat. No. 3,832,027. Subsequent developments (e.g. Spierings W.et al., “Development of an Office Holoprinter II”, SPIE Vol. 1667Practical Holography VI 1992) have replaced the photographic film usedin U.S. Pat. No. 3,832,027 with an LCD screen.

An alternative scheme to the “2-step” printing process is described inU.S. Pat. No. 4,206,965 whereby the photographic images are directlymultiplexed onto the final white-light viewable hologram in the form ofmany long thin slit holograms located side by side, thereby avoiding theneed for creating an intermediate H1 hologram. Holographic printingschemes in which the final white-light-viewable hologram is printeddirectly without the need to generate an intermediate (H1) hologram aregenerally referred to as “1-step” or direct-write methods. Subsequent tothis, a system was developed as described in U.S. Pat. No. 4,498,740 forthe recording of two dimensional composite holograms composed of a twodimensional grid of separate holograms, each such hologram correspondingto a single object point. However, this latter system suffered from thedisadvantage that the image should be located very close to therecording material. Additionally, the system was unable to formholograms which faithfully reconstructed the directional properties oflight emanating from each image point.

U.S. Pat. No. 4,421,380 describes a system for producing 1-stepfull-colour transmission holograms from 3 interlaced strip or pointcomposite holograms of the achromatic type by the inclusion of aregistered colour-filter mask. U.S. Pat. No. 4,778,262 describes a1-step method for writing directly a two dimensional matrix of basicholograms from computer data. Reference is also made to U.S. Pat. Nos.4,969,700 and 5,793,503. U.S. Pat. No. 5,138,471 describes a similartechnique whose preferred embodiment used a one dimensional spatiallight modulator connected to a computer to directly write (1-step)common types of holograms as a two-dimensional matrix of basicholograms. U.S. Pat. No. 4,834,476 describes yet another similar 1-steptechnique based on computational or sequential camera data whose use wasdescribed for the direct writing of “Alcove” (curved) compositeholograms having either a reflection or transmission geometry but whichtechnique could be generalized to more conventional flat holograms.

Perhaps the most pertinent prior-art with regards 1-step direct-writeholographic printers is the work of Yamagushi et al. (“Development of aprototype full-parallax holoprinter”, Proc. Soc. Photo-Opt Instrum. Eng(SPIE) vol. 2406, Practical Holography IX, pp 50–56 February 1995 and“High Quality recording of a full-parallax holographic stereogram withdigital diffuser”, Optical Letters vol 19, no 2 pp 135–137 Jan. 20,1994). This is discussed in more detail below and the known arrangementis described making reference to FIG. 16. A CW HeNe laser 1601 producesa beam which traverses an acousto-optic modulator 1602 before beingrelayed by mirrors 1603, 1604 and 1605 to the beam splitter 1609. Thefunction of element 1602 is to act a simple shutter. At element 1609 thebeam is broken into a reference arm and an object arm. The object beampasses through a ½ wave plate 1608 and a polarizer 1607 for polarizationadjustment. It is then redirected by mirror 1606 before passing throughtelescope lenses 1612 and 1613. The beam is now steered by mirror 1614to illuminate a twisted-nematic LCD panel 1615 having a resolution of340×220 pixels with optional attached pseudorandom diffuser 1616 beforebeing converged to a small spot of size 0.3 mm×0.3 mm on aphotosensitive film 1620 within a defining aperture 1618 with a plungingmechanism 1619 for clamping said aperture and film together at eachexposure.

The reference beam produced by element 1609 traverses the ½ waveplate1610 and polarizer 1611 before being directed, via mirror 1621, onto thephotosensitive substrate 1620 at the location defined by the aperture1622, said aperture matching aperture 1618 but located on the referencebeam side of the film.

The above system thus causes a reference beam and an object beam toco-illuminate a photosensitive film from opposite sides of said film ina small zone known as a holographic pixel or holopixel. The size of theholographic pixel thus made is effectively determined by the apertures1618 and 1622. The object beam is focused down to said holographic pixelby the lens 1617 whose Fourier plane is arranged to lie on thephotosensitive material 1620. By moving the photosensitive film 1620 ina two dimensional stepped manor and at each step changing the image inthe LCD 1615, waiting for the system vibration to die out and thenexposing a subsequent holopixel, a plurality of such holopixels arerecorded onto the photosensitive film 1620. By computationallycalculating all required LCD images a monochromaticwhite-light-reflection hologram is thus generated of a 3-D full parallaxscene or object.

The above arrangement suffers from many disadvantages. Foremost the useof a CW laser severely limits the write time of each holographic pixel.In addition air currents, temperature changes and environmental soundwill generally disturb the proper operation of such a printer. Hence,the arrangement suffers from a low printing speed, and it is notpractically possible to implement such a device outside of a strictlycontrolled laboratory environment. It is to be noted, for example, thatit is disclosed to take around 36 hours in order to write even a smallhologram of 320×224 holopixels.

Another disadvantage of the above system is that it can only produceholographic pixels of one size. This is because both contact apertures1618, 1622 and the fixed pseudorandom diffuser 1616 of pitch equal tothat of the LCD are used to define the size of said holographic pixels.Both of these subsystems fundamentally constrain the holographic pixelsize. Such a system is not therefore able to continuously change theholographic pixel size and hence different formats of holograms whichrequire fundamentally different pixel sizes can not be readily produced.

The use of contact apertures 1618, 1622 in the system, apart from beinginflexible, is also highly problematic since the emulsion surface of thephotosensitive material is very sensitive.

Another disadvantage of this arrangement is that it is only designed toproduce monochromatic reflection type holograms. Therefore transmissiontype holograms such as rainbows and achromats are precluded. The systemis also unable to produce master H1 type holograms, and is similarlyincapable of producing any form of multiple colour hologram.

Another disadvantage of the above system is that the wide-angleobjective 1617 employed is designed to only minimize sphericalaberration, is simplistic in design and only allows a restricted set ofholographic formats to be produced.

Another disadvantage of the system is that the reference beam angle isfixed and cannot be controlled as may be required, for instance, toarrange for different hologram replay conditions. This is particularlyproblematic at large format.

As is readily apparent, the above described holographic printer suffersfrom numerous problems which render it impractical to use commercially.

In many cases the 2-step method of generating an intermediate H1hologram from computer data and then copying or image-plane transferringthis hologram to form a white-light viewable hologram (H2) is to bepreferred over the above mentioned methods of directly writing the finalhologram. This is due to a number of reasons. Firstly, it is frequentlypreferred to generate restricted parallax holograms, having onlyhorizontal parallax. With the 2-step technique which produces anintermediate H1 hologram, such an H1 hologram may essentially becomposed of one or more one-dimensional strips of overlappingholographic pixels. The classical optical transfer technique then takescare of the much harder computational step of calculating thedistribution of light over the entire two-dimensional surface of thefinal (H2) hologram. If such a final hologram is written directly as ina 1-step printing scheme then this computation must be done by computer.In addition, for large holograms, the time required to write a twodimensional array of holographic pixels is usually proportional to thesquare of the time required to % write the H1 master hologram and assuch can become prohibitively long for some applications. Furthermore, afrequent complaint of directly written 1-step composite holograms isthat the holograms appear “pixelated” whereas the 2-step technique ofusing an H1 master hologram is less prone to this problem.

Notwithstanding the above, there are many situations where it isadvantageous to directly write the final hologram by a 1-stepdirect-write method. For example, directly written holograms are moreeasily tiled together to form ultra-large displays. Also in manyapplications quick previews of the final hologram are required and it isnot generally convenient to produce an H1 hologram and then to put thishologram into another machine in order to generate the final H2hologram. Additionally, the 1-step technique of directly writingholograms allows the creation of hybrid holograms having verynon-standard viewing windows, something which is likely to be demandedby the printing industry in the context of holographic bill-boarddisplays. Further advantages of the 1-step system are that materialssuch as photopolymers (see for example European patent EP0697631B1) maybe used which require only dry processing, whereas the more sensitiveSilver Halide materials requiring wet processing must be employed forclassically copied H2 holograms due to simple energy considerations.

Known 1-step and 2-step holographic printing processes employ CW lasersand thus, as a result, conventional holographic printing technology hasbeen fundamentally slow and prone to vibrational disturbance.

In order to examine the salient features of the known 2-step holographicprinters, the holoprinter described by U.S. Pat. No. 3,832,027 isreproduced in FIG. 15 and will be discussed below. A CW laser 41emitting a monochromatic beam 71 is steered by prism 62 towards a beamsplitter 43. Here the beam is divided into two parts. One part iscommonly known as the reference beam and the other part as the objectbeam. The reference beam then further travels to a spatial filter andcollimator (46 to 48) thus producing a collimated beam 72 which issteered by mirror 64 to an overhead tilted mirror 65 which finallydirects said beam onto a photosensitive substrate 60 from above and at asuitable angle. A thin vertical aperture 58 covers the photosensitivesubstrate 60 in order to mask all but a thin vertical stripe 59 in saidsubstrate.

The object beam emanating from optic 43 is reflected by prism 63 to aprojection system 51 consisting of illumination lens 52, a photographicfilm transparency advance system 53 with film image 33 and a projectionlens 54. The purpose of this projection system 51 is to project amagnified and focused image of the image, present on the film frame 33,onto the large diffusion screen 56 in coherent light. The light fromthis magnified image is then diffused by the diffuser in a wide varietyof directions with some of said light falling onto the area of thephotosensitive substrate 59 not covered by the aperture 58.

The system works by moving, in steps, the aperture across thephotosensitive material surface in a direction orthogonal to the slitdirection (ie vertically in the diagram and horizontally in reality) andby a finite amount, making a laser exposure at each such step. The filmadvance system is operated each time the aperture is moved such that thefilm image is changed at each exposure. By arranging that a set ofappropriate perspective views of a certain 3-D scene or object arestored on the film roll, a holographic stereogram may thus be encoded onthe photosensitive substrate 60.

There are many disadvantages of this system. Foremost, the use of a CWlaser means that the entire system must be installed on a vibrationisolation platform which must usually be pneumatically suspended. Inaddition air currents, temperature changes and environmental sound willgenerally disturb the proper operation of such a printer. Hence thesystem suffers from a low printing speed and it is impractical to usesuch a device outside of a strictly controlled laboratory environment.

Another disadvantage of this holoprinter is that a diffusion screen isutilized onto which 2-D perspective view images are projected. When theH1 hologram produced by this method is transferred to form an H2hologram that is white-light viewable (see e.g. FIG. 6 of U.S. Pat. No.3,832,027), the size of such final white-light viewable hologram (H2)must be less than or equal to the size of the diffusion screen 56. Thus,for example, if it is desired to generate a 1 m×1 m white light viewablehologram then a diffusion screen of at least 1 m×1 m size must be used.Since the distance D shown in FIG. 15 must correspond to both the finaloptimum viewing distance of the white-light-viewable hologram and thedistance D, shown in FIG. 6 of U.S. Pat. No. 3,832,027, such distance Dmust usually be rather greater than the hologram size. One can thus seethat the intensity of object light finally falling through the slit 59of the aperture 58 onto the photosensitive material 60 of FIG. 15 ismany orders of magnitude less than the total light illuminating thediffusion screen. In the case that it is desired to generate awhite-light-viewable hologram (H2) of size 1 m×1 m by the processdescribed in FIG. 6 of U.S. Pat. No. 3,832,027, a value of D shown inFIG. 15 of approximately 1 m may sensibly be adopted. Taking the averagesensitivity of standard Silver Halide holographic film to be 50 μJ/cm²and making various realistic system approximations it can be shown thata minimum laser energy of 1 Joule is required. Therefore, in order towrite such holograms, either a large CW laser would be required or verylong exposures must be used. However a powerful laser is undesirable dueto the problems of thermal heating of the various optical components,particularly the film 33, which must remain interferometrically staticduring each and every exposure. Long exposure times are undesirablebecause of problems due to vibration.

Another disadvantage of the above system is that a diffusion screen,aside from being energetically inefficient, inevitably deteriorates theimage quality.

Another disadvantage of the above system is that a point source is usedto illuminate the film transparency and thus the final image fidelitywill be severely limited.

Another disadvantage of the above system is that a large moving aperturemust move in quasi-contact with the photosensitive emulsion surface.This is usually very problematic as the emulsion of the photosensitivematerial 60 is usually highly fragile and yet if the aperture 58 is heldat more than a very small distance from said emulsion surface then thequality of the generated hologram will rapidly fall.

A yet further disadvantage of the above arrangement is that the movingaperture will inevitably leave areas of the hologram which are eitherdoubly exposed or unexposed, thus diminishing the quality. This isparticularly true when the slit size 59 is much smaller than thehologram size.

Another disadvantage of the above arrangement is that it is only capableof making H1 type holograms and cannot directly write 1-stepwhite-light-viewable holograms where the 3-D object bisects the hologramplane.

A further disadvantage of the above arrangement is that it is onlycapable of reasonably writing single parallax holograms asgeneralization of the technique to full parallax would render thetechnique hopelessly cumbersome given the above cited problems. Acommercial holographic printing device must be expected to be relativelycompact, operate in a normal commercial environment which is prone tovibrations, produce a variety of hologram formats and possess reasonableprint times.

Accordingly it is desired to provide an improved holographic printer.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided aholographic printer for writing H1 master holograms for subsequentconversion to white-light viewable holograms, comprising:

-   -   a laser source arranged to produce a laser beam at a first        wavelength;    -   a lens system for writing a master hologram comprising a        plurality of holographic pixels on to a photosensitive medium;    -   positioning means for positioning the photosensitive medium        relative to said lens system;    -   wherein:    -   the laser source comprises a pulsed laser source;    -   the positioning means is arranged and adapted to position the        photosensitive medium at a position downstream of the Fourier        plane and upstream of the image plane of the lens system; and    -   the holographic printer further comprises automatic spatial        coherence varying means for automatically varying the spatial        coherence of the laser beam so as to control the diameter of the        object laser beam at the Fourier plane. Preferably, the        automatic spatial coherence varying means can control in a        continuously variable manner the diameter of the object laser        beam at the Fourier plane.

The step of using a pulsed laser as the laser source of a holographicprinter is particularly advantageous since it enables the printer tooperate without sensitivity to external or internal vibration or slighttemperature fluctuations. In addition the speed of printing isfundamentally increased as there is no need to wait for vibration todissipate before making an exposure. Thus the write speed is essentiallydetermined by the refresh rate of the SLM used. Accordingly thepreferred embodiment can work several orders of magnitude faster thanconventional printers which use a CW laser and with a reliability ofoperation fundamentally higher.

The positioning of the photosensitive material, in use, at a positiondownstream of the Fourier plane and upstream of the image plane of thelens system should be contrasted to the above mentioned known systemwhere a lens system is used to project an image onto a diffusion screenwhich then subsequently scatters light onto the photosensitive material.In the preferred embodiment the photosensitive material is placed, inuse, nearer the Fourier plane than the Image plane. As a result of this,an energy economy of at least several orders of magnitude is providedwhich enables a much smaller laser source to be used than that utilisedin conventional holographic printers. Secondly, the preferred embodimentallows a compact machine to be constructed in contrast to conventionaldevices where a larger hologram size requires a proportionally largerprinter. Thirdly, by directly exposing the photosensitive material abetter image quality is attained. Fourthly, by directly exposing thephotosensitive material the awkward apertures of the prior art arerendered redundant.

The fact that the holographic printer further comprises an automaticspatial coherence varying means for automatically varying the spatialcoherence of the laser beam allows the diameter of the object laser beamat the Fourier plane to be controlled. Generally the larger the diameterof the object laser beam at the Fourier plane the greater the fidelityof the final image. However, if the beam diameter at the Fourier planebecomes too large, hologram image depth will be lost. Since the optimumsize of the diameter of the object beam at the Fourier plane is afunction of the type of hologram being written, the format of saidhologram, the image contained therein and various other printerparameters, it is highly desirable to be able to continuously changethis diameter.

Preferably, the automatic spatial coherence varying means comprises anadjustable telescope and a microlens array, wherein the adjustabletelescope is arranged to create an approximately collimated variablediameter laser beam that illuminates the microlens array. The telescopeis arranged to illuminate a variably controllable area of the microlensarray and the lenslet pitch of the lens array may be chosen such thatindividual lenses emit radiation that substantially does not superposeto create speckle. Thus it is possible to effectively and simply controlthe diameter of the object beam at the Fourier plane and also to createa high fidelity image of the LCD screen effectively illuminated by theensemble of radiative lenslet sources and substantially free of speckle.

Preferably, the holographic printer further comprises a translatablespatial light modulator arranged downstream of the automatic spatialcoherence varying means and upstream of the lens system. Currentlyavailable spatial light modulators have finite resolution. In order toattain a higher final hologram resolution than is otherwise possiblewith a static SLM, the spatial light modulator may be moved within theinput data plane of the objective. Such a system increases the effectiveholographic resolution capabilities of the holographic printer.

Preferably, the holographic printer further comprises means formodifying images sent to the spatial light modulator so as to at leastpartially correct for inherent optical distortions of the printer.Software correction of the digital computer images prior to theirdisplay on the spatial light modulator is a highly desirable preferredfeature of the present invention. This is because, in order to designsuitable wide angle objectives for a holographic printer, betterperformance in eliminating aberrations characterised by the first fourSeidel coefficients may be realised if some optical distortion (5coefficient) is accepted. Thus effectively a better objective limitingresolution and a better objective field of view may be attained in thecase that the wide angle objective possesses some barrel or pincushiondistortion. Since, for many types of hologram, different colour channelsmust be written which must exactly register, the use of software imagecorrection is particularly advantageous.

Preferably, said lens system has an effective field of view greater than70 degrees, preferably greater than 75 degrees, further preferablygreater than 80 degrees, further preferably at best 85 degrees. The lenssystem's field of view determines the maximum field of view possible fora final image-planed white-light hologram produced from the H1 mastersof the preferred embodiment. It also determines the format of hologramsthat a holographic printer can produce. Thus a lens system of field ofview under 70 degrees would severely limit the application of thedevice.

Preferably, the Fourier plane of the lens system is located downstreamof the lens system, further preferably at least 1 mm, 1.5 mm, 2 mm or2.5 mm downstream of the lens system. It is a difficult task to design awide angle objective that has its minimum waist (Fourier Plane) outsideand downstream of the objective. It is an even more difficult task toallow sufficient space between the final lens of the objective and thisplane such that a reference beam may be brought in (from the objectiveside) at Brewster's angle to co-illuminate a photosensitive film near orat the Fourier plane (see e.g. FIG. 12). If the distance from theFourier plane to the objective is much less than about 2 mm then itbecomes virtually impossible to utilise currently available spatiallight modulators.

Preferably, the laser source is arranged to additionally produce laserbeams at second and third wavelengths, the first, second and thirdwavelengths each differing from one another by at least 30 nm. Byarranging for the laser source to be a multiple colour multiple colourmaster holograms may be constructed which may be used to producemultiple colour image planed holograms.

Preferably, the holographic printer further comprises a second and athird laser source for producing laser beams at second and thirdwavelengths, the first, second and third wavelengths each differing fromone another by at least 30 nm. An alternative arrangement to a multiplecolour laser is several lasers each producing a different colouremission.

Preferably, the holographic printer further comprises a first lenssystem for use at the first wavelength, a second lens system for use atthe second wavelength, and a third lens system for use at the thirdwavelength, wherein the first, second and third lens systems arearranged so that a desired lens system may be automatically selected. Asmentioned above the design of the objective is critical and usually afar better objective may be designed if it is to function only at onewavelength. Hence when using multiple colour operation different lenssystems are preferably used which are optimised to one particularwavlength.

According to a second aspect of the present invention, there is provideda holographic printer for writing master holograms for subsequentconversion to white-light viewable holograms, comprising:

-   -   a laser source arranged to produce a laser beam at a first        wavelength;    -   a lens system for writing a master hologram comprising a        plurality of holographic pixels on to a photosensitive medium;    -   positioning means for positioning the photosensitive medium        relative to the lens system;    -   wherein:    -   the laser source comprises a pulsed laser source; and    -   the positioning means is arranged and adapted to position the        photosensitive medium at a position downstream of the Fourier        plane and upstream of the image plane of the lens system.

Preferably, the holographic printer further comprises spatial coherencevarying means for varying the spatial coherence of the laser beam, thespatial coherence varying means comprising a plurality of discretediffractive elements and wherein the number of discrete diffractiveelements illuminated by the laser beam may be varied and/or controlled.

Preferably, the holographic printer further comprises a plurality oflenslets and means for varying the number of lenslets illuminated by thelaser beam.

Preferably, the holographic printer further comprises means for varyingthe spatial coherence of the beam, further preferably in an automaticmanner, without substantially introducing speckle noise.

Preferably, the holographic printer further comprises means for varyingthe spatial coherence of the laser beam in a continuously variablenon-discrete manner.

Preferably, the holographic printer further comprises spatial coherencevarying means comprising a plurality of components, wherein therelationship between the components may be varied in order to change thespatial coherence of the laser beam. According to a particularlypreferred embodiment, the relationship which may be varied is therelative distance between two lenses.

According to a third aspect of the present invention there is provided amethod of writing master holograms for subsequent conversion towhite-light viewable holograms, comprising:

-   -   providing a laser source arranged to produce a laser beam at a        first wavelength;    -   providing a lens system for writing a master hologram comprising        a plurality of holographic pixels on to a photosensitive medium;    -   positioning a photosensitive medium relative to the lens system;    -   wherein:    -   the laser source comprises a pulsed laser source;    -   the photosensitive medium is positioned downstream of the        Fourier plane and upstream of the image plane of the lens        system; and    -   the method further comprises the step of:    -   automatically varying the spatial coherence of the laser beam so        as to control in a continuously variable fashion the diameter of        the object laser beam at the Fourier plane.

According to a preferred embodiment there is provided a single methodand apparatus capable of both (i) writing directly a final white-lightviewable composite holographic stereogram and (ii) writing an H1 masterhologram that can be used to then generate a white-light viewableholographic stereogram by classical image planing, the stereograms beingeither of restricted parallax or of full parallax and of single colouror of multiple colour.

The preferred embodiment solves the problem of the sensitivity toambient and machine-caused vibration in a commercial holographicprinting machine by the use of a pulsed laser having appropriatetemporal and spatial beam characteristics, such that the writing time ofthe hologram is limited only by the refresh rate of the spatial lightmodulator employed.

The preferred embodiment employs the combination of a spatial lightmodulator, an aberration minimized wide-angle objective having a minimumbeam waist outside the objective and uses a method of controlling thespatial coherence of the laser beam passing through said opticalelements without the induction of significant speckle noise, in order torecord an H1 master hologram without use of a diffusion screen ontowhich an image is conventionally projected.

The preferred embodiment uses the combination of a spatial lightmodulator, an aberration minimized wide-angle objective having a minimumbeam waist outside the objective and uses a method of controlling thespatial coherence of the laser beam passing through said opticalelements without the induction of significant speckle noise, in order torecord a general composite hologram, which may or may not be white-lightviewable, in which the pixel size of the individual component hologramsis controlled preferably in a continuous fashion by such spatialcoherence and in which the light intensity distribution of such pixel isfavorable.

According to the preferred embodiment the spatial light modulator, whichdisplays the individual images for each holographic pixel, may be movedin a one or two dimensional fashion within the input data plane of theabove said objective between individual exposures such that higherresolution images may be attained for H1 master holograms.

Preferably, the spatial light modulator, which displays the individualimages for each holographic pixel, may be moved in a one or twodimensional fashion within the input data plane of the objective betweenindividual exposures such that a well-defined rectangular hologramviewing window may be created when directly writing a 1-step hologram.

According to another embodiment the spatial light modulator may remainfixed within the objective pupil and that any required translation (bethis side-to-side or up/down) of the displayed image between individualexposures be accomplished by software.

Preferably, a combination of software image displacement and mechanicalmovement of the spatial light modulator may be used to attain aneffective image translation in the input data plane of the objective.

Preferably, in the case of the generation of an H1 hologram, the elementthat controls the spatial coherence may be moved in a random or in anordered fashion in such a way as to average out and reduce any patternor spatial noise produced by such element in particular but also toreduce any other such optical noise arising in the system, thusimproving the quality of holographic image.

According to a preferred feature, in the case of the generation of an H1hologram, the packing density and size of the holographic pixels may bechosen, optimized or controlled in such a way as to average out orreduce any unwanted optical pattern or spatial noise thus improving thequality of holographic image.

In the case of the generation of an H1 hologram it is preferred tooptimize the laser output energy and the reference/object energysplitting ratio according to the packing density and size of theholographic pixels, thus improving the quality and brightness ofholographic image.

In the case of die direct generation of the final hologram, it ispreferred to choose, optimize or (continuously) control the packingdensity and size of the holographic pixels in such a way as to optimizethe quality, brightness and depth of field of the holographic image.

In the case of the generation of an H1 hologram it is preferred to causethe reference beam distribution to track automatically the object beamin shape and position at the holographic film plane by an appropriatemeans such as a computer-controlled motorized image-planed variablymagnified aperture.

Preferably, when a colour pulsed laser is used to produce eithermultiple-colour H1 holograms or multiple-colour 1-step holograms, threeseparate optical systems having controllable write-location of at leastone of the optical wide-angle objectives are implemented permitting thewriting of holographic pixels of different colour onto a panchromaticemulsion in parallel, such that either the holographic pixels ofdifferent colour can be made to line up or that the holographic pixelsof different colour can be made to form a specific and controllablepattern.

Preferably, when a colour pulsed laser is used to producemultiple-colour H1 holograms, one multi-wavelength optical system isused and various wavelength critical elements in this optical system arereplaced and selected automatically between exposures of differentcolours.

Data to write the holograms are preferably either generated by a 3-Dcomputer model or are taken from a plurality of sequential camera shots.The holograms are recorded by laser light onto a suitable recordingmedium by means of a write head employing a spatial light modulatorattached to a computer. The recording material or the write head ismoved in a one or two-dimensional sense in order to write an array ormatrix comprising a plurality of pixels.

According to an aspect of the present invention there is provided aholographic printer incorporating a pulsed laser for writing digitalmaster holograms (H1).

According to an aspect of the present invention there is provided aholographic printer designed to print digital H1 master holograms andincorporating a pulsed laser, a SLM, an ultra-wide angle objective and ameans for variably controlling the object beam spatial coherence.

Preferably, the SLM is static and effectively fills the input data planeof the ultra-wide angle objective.

Preferably, the SLM is moved, from one holopixel exposure to another, ina one or two-dimensional fashion in the input data plane of the wideangle objective.

Preferably, the wide angle objective has one or more of the followingproperties: (a) it is designed to work at a specific wavelength; (b) ithas a beam waist significantly outside the objective; (c) it has lowoptical aberration and high resolution; (d) it has an effective field ofview greater than 70 degrees; and (e) it has significant opticaldistortion (i.e. aberration described by the 5^(th) Seidel coefficient)requiring software (SLM) image correction.

Preferably, the method of variably controlling the object beam spatialcoherence consists of using an adjustable telescope (creating anapproximately collimated variable diameter laser beam) that illuminatesa microlens array.

Preferably, the pulsed laser is a monochromatic pulsed laser having apulse duration between 1 femtosecond and 100 microseconds and a temporalcoherence of greater than 1 mm.

Preferably, the pulsed laser is a Neodymium laser that is furtherpreferably either flashlamp or diode pumped.

Preferably, the pulsed laser is a multiple colour laser having a pulseduration of each colour component between 1 femtosecond and 100microseconds and a temporal coherence of each colour component greaterthan 11 mm.

Preferably, the holographic pixel size of any produced hologram isoptimized and controlled to achieve the best image fidelity.

Preferably, at least some of the electromechanical translation androtation stages employed therein are controlled by a special controllerthat allows constant velocity and non-linear movement trajectories ofsaid electromechanical stages to be programmed, thus assuring the smoothand proper precise movement of at least several such stages at highrates of exposure.

Preferably, the SLM is a high resolution LCD.

Preferably, software image distortion algorithms are applied for eachholographic pixel written in order to correct for the inherent opticaldistortion in the optical system of the printer and to assure anon-distorted hologram replay image under a certain final illuminationlight geometry.

Preferably, the method of variably controlling the object beam spatialcoherence is arranged so as not to induce significant speckle noise intothe final hologram.

Preferably, software image distortion algorithms are applied to eachimage sent to the SLM, the exact form of such distortions beingcalculated with reference to the position of the SLM in the objectiveinput data plane and the holographic pixel being written, in order tocorrect for the inherent optical distortion in the optical system of theprinter and to assure a non-distorted hologram replay image under acertain final illumination light geometry.

According to a further aspect of the present invention there is provideda digital holographic printer, designed to print digital H1 masterholograms, incorporating a pulsed laser, one or more SLMs, one or morewide angle objectives and a method for variably controlling the spatialcoherence of each object beam.

According to a further aspect of the present invention there is provideda digital holographic printer, designed to print digital H1 masterreflection holograms, incorporating a multiple colour pulsed laser, 3 ormore SLMs, 3 or more wide-angle objectives, a means for variablycontrolling the spatial coherence of each object beam and a means ofvariably adjusting the spacing between holopixels of different colour.

According to a further aspect of the present invention there is provideda digital holographic printer, designed to print digital H1 masterreflection holograms, incorporating a multiple colour pulsed laser, 3 ormore SLMs, 3 or more wide-angle objectives, a means for variablycontrolling the spatial coherence of each object beam and where thespacing between holopixels of different colour is fixed and may or maynot be zero.

According to a further aspect of the present invention there is provideda digital holographic printer, designed to print digital H1 masterreflection holograms, incorporating a multiple colour pulsed laser, 1SLM, 3 or more wide-angle objectives that can be automatically ormanually inserted into or retracted from a critical position in oneprincipal optical circuit and a means for variably controlling thespatial coherence of the object beam, such holographic printer printingsequentially in one colour and then making another pass for the nextcolour.

According to a further aspect of the present invention there is provideda digital holographic printer, designed to print digital H1 masterreflection holograms, incorporating a colour pulsed laser where onecolour channel is written first after which the printer makes anotherpass writing the next colour and so forth, such passes either being anentire print line, part of a print line, a region to be printed or theentire region to be printed.

According to a further aspect of the present invention there is provideda digital holographic printer, designed to print digital H1 masterreflection holograms, incorporating a colour pulsed laser where one ormore colour channels are written at the same time.

According to a further aspect of the present invention there is provideda digital holographic printer, designed to print digital H1 masterreflection holograms, incorporating a colour pulsed laser where one ormore optical elements are replaced by holographic optical elements.

According to a further aspect of the present invention there is provideda digital holographic printer, designed to print digital H1 masterreflection holograms, incorporating a pulsed laser where one or moreoptical elements are replaced by holographic optical elements.

According to a further aspect of the present invention there is provideda digital holographic printer, designed to print digital H1 mastertransmission holograms, incorporating a colour pulsed laser of 3 or morecolours, where 3 or more master holograms, one for each colour, isrecorded on a different holographic film or plate.

According to a further aspect of the present invention there is provideda digital holographic printer, designed to print digital H1 mastertransmission holograms, incorporating a colour pulsed laser of 3 or morecolours, 3 or more SLMs. 3 or more wide-angle objectives, a method forvariably controlling the spatial coherence of each object beam where 3or more master holograms, one for each colour, is recorded on adifferent holographic film or plate.

According to a further aspect of the present invention there is provideda digital holographic printer, designed to print digital H1 mastertransmission holograms, incorporating a colour pulsed laser of 3 or morecolours, 1 SLM, 3 or more wide-angle objectives that can beautomatically or manually inserted into or retracted from a criticalposition in one principal optical circuit and a method for variablycontrolling the spatial coherence of the object beam, where 3 or moremaster holograms, one for each colour, is recorded on a differentholographic film or plate.

According to an embodiment software image distortion algorithms areapplied to each image sent to the SLM in order to correct for theinherent optical distortion in the wide angle objective of the printer.

According to a preferred feature an image-planed aperture is used tocontrol the size and shape of the reference beam. Preferably, saidaperture is moved in a one or two dimensional fashion in order toaccurately modify the location of the reference beam on the holographicfilm plane. Preferably, the reference beam is made to automaticallytrack the object beam at the holographic film plane. Preferably, thedistance of the objective from the holographic film plane is controlledso as to change the holographic pixel size.

Preferably, the spatial coherence of the object beam is controlled tochange and optimize the diameter of the object beam at the location ofits minimum beam waist after passing through the wide angle objective.

Preferably, image fidelity of the hologram is further optimized bychoosing the spatial density of holographic pixels written. Furtherpreferably the density is varied from region to region in the hologram.

Preferably, the element that controls the spatial coherence of theobject beam is moved in a random or particular fashion between holopixelexposures so as to diminish any noise that would otherwise deterioratethe quality of the written hologram.

Preferably, the SLM is moved toward and away from the wide angleobjective on a precision stage so as to control the optimum H1–H2transfer distance when the master hologram is finally transferred to anH2 hologram.

Preferably, in the case of a static SLM, the required image translationwithin the input data plane of the objective is accomplished bysoftware.

Preferably, an H1 master hologram is written which is designed to beimage-plane transferred to create a single colour channel rainbowhologram, said H1 master hologram consisting of a single line ofstrongly overlapping holographic pixels.

Preferably, an H1 master hologram is written which is designed to beimage-plane transferred to create a multiple colour channel rainbowhologram, said H1 master hologram consisting of a several lines,vertically displaced, of strongly overlapping holographic pixels.

Preferably, an H1 master hologram is written which is designed to beimage-plane transferred to create a single colour channel reflectionhologram of either full or horizontal parallax, the master hologramconsisting of a 2-dimensional matrix of strongly overlapping holographicpixels.

Preferably, an H1 master hologram is written which is designed to beimage-plane transferred to create a single colour channel reflectionhologram of either full or horizontal parallax, the master hologramconsisting of a 2-dimensional matrix of strongly overlapping holographicpixels whose density is a general function of the Cartesian pixelcoordinates on the holographic substrate.

Preferably, an H1 master hologram is written which is designed to beimage-plane transferred to create a single parallax singlecolour-channel reflection hologram, the H1 master hologram consisting ofa 2-dimensional matrix of strongly overlapping holographic pixels whosedensity in the vertical and horizontal directions is not the same.

Preferably, an H1 master reflection hologram is written which isdesigned to be image-plane transferred to create a multiple colourreflection hologram of either full or horizontal parallax, the H1 masterhologram consisting of a 2-dimensional matrix of strongly overlappingholopixels.

Preferably, an H1 master reflection hologram is written which isdesigned to be image-plane transferred to create a single parallaxmultiple colour-channel reflection hologram, the H1 master hologramconsisting of a 2-dimensional matrix of strongly overlapping holographicpixels whose density in the vertical and horizontal directions is notthe same.

Preferably, an H1 master reflection hologram is written which isdesigned to be image-plane transferred to create a single or fullparallax multiple colour-channel reflection hologram, the H1 masterhologram consisting of a 2-dimensional matrix of strongly overlappingholographic pixels whose density is a general function of the Cartesianpixel coordinates on the holographic substrate.

Preferably, an H1 master reflection hologram is written which isdesigned to be image-plane transferred to create a single or fullparallax multiple colour-channel reflection hologram, the H1 masterhologram consisting of a 2-dimensional matrix of weakly overlappingholographic pixels whose density is a general function of the Cartesianpixel coordinates on the holographic substrate.

Preferably, an H1 master reflection hologram is written which isdesigned to be image-plane transferred to create a single or fullparallax multiple colour-channel reflection hologram, the H1 masterhologram consisting of a 2-dimensional matrix of weakly overlapping orabutting holographic pixels, interlaced by colour and whose density is ageneral function of the pixel Cartesian coordinates on the holographicsubstrate.

Preferably, an H1 master hologram is written which is designed to beimage-plane transferred to create a either a multiple colour channelrainbow hologram or an achromatic transmission hologram, each holopixelbeing written by an object beam whose propagation vector is parallel tothe normal vector of the holographic film.

Preferably, an H1 master hologram is written which is designed to beimage-plane transferred to create a either a multiple colour channelrainbow hologram or an achromatic transmission hologram, each holopixelbeing written by an object beam whose propagation vector makes an angleto the normal vector of the holographic film. Further preferably, saidangle is the achromatic angle.

According to an embodiment there is provided a digital holographicprinter designed to print digital H1 master holograms, incorporating apulsed laser, multiple SLMs, multiple wide angle objectives, a method ofvariably controlling the spatial coherence of each object beam and amethod of variably adjusting the spacing between holopixels written byeach wide-angle objective.

According to an embodiment there is provided a digital holographicprinter designed to print digital H1 master holograms, incorporating amultiple colour pulsed laser, multiple SLMs, multiple wide angleobjectives, a method of variably controlling the spatial coherence ofeach object beam and a method of variably adjusting the spacing betweenholopixels written by each wide-angle objective.

According to a further aspect of the present invention there is provideda holographic printer for directly writing 1-step white-light viewableholograms comprising:

-   -   a laser source arranged to produce a laser beam at a first        wavelength;    -   a lens system for directly writing a hologram comprising a        plurality of holographic pixels on to a photosensitive medium;    -   positioning means arranged and adapted to position the        photosensitive medium substantially at the Fourier plane of the        lens system;    -   wherein:    -   the laser source comprises a pulsed laser source; and    -   the holographic printer further comprises automatic spatial        coherence varying means for automatically varying the spatial        coherence of the laser beam so as to control in a continuously        variable fashion the diameter of the object laser beam at the        Fourier plane.

The step of using a pulsed laser as the laser source of a holographicprinter is particularly advantageous since it enables the printer tooperate without sensitivity to external or internal vibration or slighttemperature fluctuations. In addition the speed of printing isfundamentally increased as there is no need to wait for vibration todissipate before making an exposure. Thus the write speed is essentiallydetermined by the refresh rate of the SLM used. Accordingly thepreferred embodiment can work several orders of magnitude faster thanconventional printers which use a CW laser and with a reliability ofoperation fundamentally higher.

The positioning of the photosensitive material, in use, at substantiallythe Fourier plane is optimal as at any other plane significant holopixeloverlap on the surface of the photosensitive material would be requiredin order not to produce apparently sparsely pixelated holograms. This isbecause light rays cross over each other at the Fourier plane. Thus, inthe case that the Fourier plane is at a distance, L, from thephotosensitive material, the final image will appear to be made up ofholopixels, located at this distance L from the photosensitive material.The apparent width of these pixels will be equal to the object beamdiameter at the Fourier plane which is always less than the size of theobject beam at the surface of the photosensitive material. Thus, in thecase that the photosensitive material is not substantially at theFourier plane, in order to properly abut neighbouring holopixelssignificant overlap of the object beam footprints of such neighbouringholopixels on the photosensitive material would be required, thusreducing the hologram diffractive efficiency.

The fact that the holographic printer further comprises an automaticspatial coherence varying means for automatically varying the spatialcoherence of the laser beam allows the diameter of the object laser beamat the Fourier plane to be controlled. This then means that the size ofthe holopixel may be controlled. Since different formats of hologramrequire fundamentally different pixel sizes it is highly desirable to beable to continuously change this diameter.

Preferably, the automatic spatial coherence varying means comprises anadjustable telescope and a microlens array, wherein the adjustabletelescope is arranged to create an approximately collimated variablediameter laser beam that illuminates said microlens array. The telescopeis arranged to illuminate a variably controllable area of the microlensarray and the lenslet pitch of the lens array may be chosen such thatindividual lenses emit radiation that substantially does not superposeto create speckle. Thus it is possible to effectively and simply controlthe diameter of the object beam at the Fourier plane and also to createa high fidelity image of the LCD screen effectively illuminated by theensemble of radiative lenslet sources and substantially free of speckle.

Preferably, the printer further comprises a translatable spatial lightmodulator arranged downstream of the automatic spatial coherence varyingmeans and upstream of said lens system. In the case that holograms areto be illuminated for display with a collimated beam of white light,translating the LCD provides a convenient and efficient way of producingholograms with rectangular viewing zones. A rectangular viewing windowis desirable, as an observer viewing the hologram will either see theentire image or no image at all. This should be contrasted to the caseof a scrolling viewing window where an observer sees, for much of thetime, only part of the holographic image. By translating the LCD ahologram with a certain rectangular viewing zone can be produced with alower resolution LCD then would otherwise be required if the LCDremained static.

Preferably, the printer further comprises means for modifying imagessent to the spatial light modulator so as to at least partially correctfor inherent optical distortions of said printer. In a preferredembodiment the holographic printer comprises means for pre-distortingimages sent to a spatial light modulator. Software correction of thedigital computer images prior to their display on the spatial lightmodulator is a highly desirable preferred feature of the presentinvention. This is because, in order to design suitable wide angleobjectives for a holographic printer, better performance in eliminatingaberrations characterised by the first four Seidel coefficients may berealised if some optical distortion (5^(th) coefficient) is accepted.Thus effectively a better objective limiting resolution and a betterobjective field of view may be attained in the case that the wide angleobjective possesses some barrel or pincushion distortion. Since, formany types of hologram, different colour channels must be written whichmust exactly register, the use of software image correction isparticularly advantageous.

In many cases, holograms are illuminated for display with anon-collimated beam of white light emanating from a point source such asa halogen lamp. If account is not taken of the replay illuminationgeometry and further a constant angle of reference is employed atrecording, both image distortion and viewing window distortion willoccur on illumination of the hologram by a diverging beam. By using acombination of image pre-distortion based on a diffractive model, one ortwo-dimensional translation of the LCD and by moving the reference beamin only one dimension at each holopixel exposure, any induced imagedistortion can be compensated for and a very much improved hologramviewing window may be attained. Thus the combination of a onedimensionally changeable reference beam, a translatable LCD and softwareimage distortion are highly desirable, particularly for largerholograms.

Preferably, the lens system has an effective field of view greater than70 degrees.

Preferably, the Fourier plane of the lens system is located downstreamof said lens system, preferably at least 1 mm, 1.5 mm, 2 mm or 2.5 mmdownstream of the lens system.

Preferably, the laser source is arranged to additionally produce laserbeams at second and third wavelengths, the first, second and thirdwavelengths each differing from one another by at least 30 nm.

Preferably, the printer further comprises a second and a third lasersource for producing laser beams at second and third wavelengths, thefirst, second and third wavelengths each differing from one another byat least 30 nm.

Preferably, the printer further comprises a first lens system for use atsaid first wavelength, a second lens system for use at said secondwavelength, and a third lens system for use at said third wavelength,wherein the first, second and third lens systems are arranged so that adesired lens system may be automatically selected.

According to a further aspect of the invention there is provided aholographic printer for directly writing 1-step white-light viewableholograms comprising:

-   -   a laser source arranged to produce a laser beam at a first        wavelength;    -   a lens system for directly writing a hologram comprising a        plurality of holographic pixels on to a photosensitive medium;    -   positioning means arranged and adapted to position the        photosensitive medium substantially at the Fourier plane of the        lens system;    -   wherein:    -   the laser source comprises a pulsed laser source.

According to a yet further aspect of the invention there is provided amethod of directly writing 1-step white-light viewable holograms,comprising:

-   -   providing a laser source arranged to produce a laser beam at a        first wavelength;    -   providing a lens system for directly writing a white-light        viewable hologram comprising a plurality of holographic pixels        on to a photosensitive medium;    -   positioning a photosensitive medium substantially at the Fourier        plane of said lens system;    -   wherein:    -   the laser source comprises a pulsed laser source; and    -   the method further comprises the step of:    -   automatically varying the spatial coherence of the laser beam so        as to control in a continuously variable fashion the diameter of        the object laser beam at the Fourier plane.

According to a further aspect of the present invention there is provideda holographic printer for directly writing 1-step white-light viewableholograms comprised of a plurality of holographic pixels, theholographic printer comprising:

-   -   a laser source arranged to produce a laser beam at a first        wavelength, the laser beam being arranged to be split into an        object beam and a reference beam;    -   a spatial light modulator arranged to operate on the object        beam;    -   a lens system for directly writing a holopixel on to a        photosensitive medium;    -   positioning means arranged and adapted to position the        photosensitive medium substantially at the Fourier plane of the        lens system;    -   wherein:    -   the laser source comprises a pulsed laser source;    -   the spatial light modulator is translatable;    -   said holographic printer further comprises:    -   means for varying in one dimension only the direction of the        reference beam at the Fourier plane after the formation of a        said holopixel;    -   automatic spatial coherence varying means for automatically        varying the spatial coherence of the laser beam so as to control        the diameter of the object laser beam at the Fourier plane; and    -   means for pre-distorting images sent to the spatial light        modulator.

According to a further aspect of the present invention there is provideda 1-step digital holographic printer incorporating a pulsed laser.

According to a further aspect of the present invention there is provideda 1-step digital holographic printer incorporating a pulsed laser, SLM,wide angle objective and a method for variably controlling the objectbeam spatial coherence. Preferably, the SLM is static and effectivelyfills the input data plane of the wide angle objective. Alternativelythe SLM is moved, from one holopixel exposure to another, in a one ortwo dimensional fashion in the input data plane of the wide angleobjective.

Preferably, the wide angle objective has one or more of the followingproperties: (a) it is designed to work at a specific wavelength, (b) ithas a beam waist significantly outside the objective, (c) it has lowoptical aberration and high resolution, (d) it has an effective field ofview greater than 70 degrees, and (e) it has significant opticaldistortion (e) aberration caused by the 5^(th) Seidel coefficient)requiring software (SLM) image correction.

Preferably, the method of variably controlling the object beam spatialcoherence consists of using an adjustable telescope (creating anapproximately collimated variable diameter laser beam) that illuminatesa microlens array.

Preferably, the pulsed laser used in the printer is a monochromaticpulsed laser having a pulse duration between 1 femtosecond and 100microseconds and a temporal coherence of greater than 1 mm.

Preferably, the pulsed laser is a Neodymium laser that is eitherflashlamp or diode pumped.

Preferably, the pulsed laser is a multiple colour laser having a pulseduration of each colour component between 1 femtosecond and 100microseconds and a temporal coherence of each colour component greaterthan 1 mm.

Preferably, the holographic pixel size of any produced hologram isoptimized and controlled for each case by changing the object beamspatial coherence.

Preferably, at least some of the electromechanical translation androtation stages employed are controlled by a controller that allowsconstant velocity and non-linear movement trajectories of saidelectromechanical stages to be programmed, thus assuring the smooth andproper precise movement of at least several such stages at high rates ofexposure.

Preferably, the SLM is a high resolution LCD.

Preferably, software image distortion algorithms are applied for eachholographic pixel written in order to correct for the inherent opticaldistortion in the wise angle objective of the printer and to assure anon-distorted hologram replay image under a certain final illuminationlight geometry.

Preferably, the method of variably controlling the object beam spatialcoherence does not induce significant speckle noise into the finalhologram.

Preferably, software image distortion algorithms are applied to eachimage sent to the SLM in order to correct for the inherent opticaldistortion in the wide angle objective of the printer and to assure anon-distorted hologram replay image under a certain final illuminationlight geometry.

Preferably, software image distortion algorithms are applied to eachimage sent to the SLM, the exact form of such distortions beingcalculated with reference to the position of the SLM in the objectiveinput data plane and the holographic pixel being written, in order tocorrect for the inherent optical distortion in the optical system of theprinter and to assure a non-distorted hologram replay image under acertain final illumination light geometry.

Preferably, when a colour pulsed laser is used to producemultiple-colour 1-step holograms, one multi-wavelength optical system isused and various wavelength critical elements in this optical system arereplaced and selected automatically between exposures of differentcolours.

According to a further aspect of the present invention there is provideda 1-step digital holographic printer incorporating a pulsed laser, oneor more SLMs, one or more wide angle objectives and a method forvariably controlling the spatial coherence of each object beam.

According to a further aspect of the present invention there is provideda 1-step digital holographic printer incorporating a multiple colourpulsed laser, 3 or more SLMs. 3 or more wide-angle objectives, a methodfor variably controlling the spatial coherence of each object beam and amethod of variably adjusting the spacing between holopixels of differentcolour.

According to a further aspect of the present invention there is provideda 1-step digital holographic printer incorporating a multiple colourpulsed laser, 3 or more SLMs, 3 or more wide-angle objectives, a methodfor variably controlling the spatial coherence of each object beam andwhere the spacing between holopixels of different colour is fixed andmay or may not be zero.

According to a further aspect of the present invention there is provideda 1-step digital holographic printer incorporating a multiple colourpulsed laser, 1 SLM, 3 or more wide-angle objectives that can beautomatically or manually inserted into or retracted from a criticalposition in one principal optical circuit and a method for variablycontrolling the spatial coherence of the object beam, such holographicprinter printing sequentially in one colour and then making another passfor the next colour.

According to a further aspect of the present invention there is provideda 1-step digital holographic printer incorporating a multiple colourpulsed laser where one colour channel is written first after which theprinter makes another pass writing the next colour and so forth, suchpasses either being an entire print line, part of a print line, a regionto be printed or the entire region to be printed.

According to a further aspect of the present invention there is provideda 1-step digital holographic printer incorporating a multiple colourpulsed laser where one or more colour channels are written at the sametime.

Preferably, an image-planed aperture is used to control the size andshape of the reference beam.

Preferably the laser energy and reference to object energy ratio ischosen so as to optimize the brightness and quality of the finalhologram.

Preferably the size of the reference beam is always matched to the sizeof the object beam at the photosensitive material surface.

Preferably, an image-planed aperture is used to control the size andshape of the reference beam whilst maintaining effective beamcollimation and low beam divergence.

According to a further aspect of the present invention there is provideda digital holographic printer, designed to print directly 1-stepholograms, incorporating a colour pulsed laser where one or more opticalelements are replaced by holographic optical elements.

According to a further aspect of the present invention there is provideda digital holographic printer, designed to print directly 1-stepholograms, incorporating a pulsed laser where one or more opticalelements are replaced by holographic optical elements.

According to an embodiment there is provided a digital holographicprinter designed to print directly 1-step digital holograms,incorporating a pulsed laser, multiple SLMs, multiple wide angleobjectives, a method of variably controlling the spatial coherence ofeach object beam and a method of variably adjusting the spacing betweenholopixels written by each wide-angle objective.

According to an embodiment there is provided a digital holographicprinter designed to print digital 1-step holograms, incorporating amultiple colour pulsed laser, multiple SLMs, multiple wide angleobjectives, a method of variably controlling the spatial coherence ofeach object beam and a method of variably adjusting the spacing betweenholopixels written by each wide-angle objective.

According to a further aspect of the present invention, there isprovided a holographic printer operable in a first mode for directlywriting 1-step white-light viewable holograms and in a second mode forwriting master holograms (H1) which are convertible by a process knownper se to white-light viewable holograms (H2). Such dual purposeholographic printers have not been previously contemplated.

In order to illustrate the differences between the embodiment directedto 2-step printing processes and conventional 2-step holographicprinting processes, reference is made to FIG. 17 which contrasts thepreferred arrangement (bottom diagram) and with conventional methods(top diagram). In the known arrangements a focused image of a spatiallight modulator 1701 is created on the diffusion screen 1703 usingobjective lens 1702. The diffusion screen scatters the impinging lightin a wide variety of directions. A photosensitive material 1705 coveredby a movable aperture 1706, which may be a general rectangle, allows theenvelope of rays delineated by 1717 and 1716 to irradiate that part ofthe surface of the material 1705 which is left uncovered by the aperturehole 1707. A mutually coherent reference beam is brought in from A to Bsuch as to create a small transmission hologram at the region 1704 whichmay be referred to as a holographic pixel. By moving the aperture in aone or two-dimensional fashion, changing the SLM image to theappropriate perspective view and by effecting an exposure, a compositetransmission hologram is built up from holographic pixels having a shapedefined by the aperture used. In the known arrangement the aperture is aslit and movement is one-dimensional. The known arrangement could begeneralized to two-dimensional movement and a rectangular or squareaperture. The resultant composite transmission hologram is thentransferred in the prior art to an H2 white-light viewable hologram byconjugate illumination of the processed hologram using a replay beam1708 whose direction of propagation is B to A. This process produces areal image at the spatial location 1703. Specifically, by covering upthe processed hologram 1705 with an aperture 1706 so as to onlyilluminate the holographic pixel 1704 by the reference beam 1708, theexact same image is now projected onto the diffusion screen 1703 by thehologram as was used to record the pixel 1704.

In the present invention no diffusion screen is used. Instead, accordingto a preferred embodiment, a highly specialized wide angle objective1714 is used to form a focused image of the SLM 1715 at the spatiallocation 1709. No material surface is present at die image plane 1709.Instead the photosensitive film 1712 is placed somewhat downstream ofthe plane of minimum beam waist as shown. The image at 1715 is generallyshifted (either by the LCD being shifted with respect to the objectivelens or by software) with respect to the image 1701 either in a onedimensional fashion or a two dimensional fashion. In this way the ray1717 in the bottom diagram corresponds exactly to the time-reversed ray1717 in the top diagram and likewise for 1718. Since there exists atime-reversal transformation between these two sets of rays a timereversed reference beam 1711 propagating from D to C is used. Thus, asabove, a holographic pixel is created at location 1713. By moving theSLM 1715, the objective lens 1714 and the reference beam 1711 togetherin a one or two dimensional (translating) fashion over the surface ofthe photosensitive film a composite transmission hologram is recorded.This hologram 1712 is conjugate to the hologram 1705. Hence if compositehologram 1712 is chemically processed and then illuminated by the samereference beam 1711 propagating from D to C, care is taken only toilluminate one holographic pixel at a time and a diffusion screen isplaced at the location 1709, the exact same images will be observed,projected onto the diffusion screen, as we retrieved using the prior artdescribed above.

Thus the preferred arrangement has many advantages over knownarrangements including an energy requirement orders of magnitude lowerthan the prior art, dramatically increased system flexibility, lowernoise results, higher speed operation and fundamentally smaller printersize.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be described, byway of example only, and with reference to the accompanying drawings inwhich:

FIG. 1 illustrates the process of acquiring data from a series ofsequential camera shots that can be used to generate the digitalholograms in addition to illustrating a computer model of an objectwhere a viewing plane is defined on which perspective views aregenerated;

FIG. 2 illustrates a plan view of a preferred embodiment of invention;

FIG. 2A is a block diagram showing one arrangement to provide multiplecolor holograms.

FIG. 3 illustrates selected key components of the preferred embodimentfrom a perspective view;

FIG. 4 illustrates an embodiment working in the H1 master writing modefor the case of a transmission H1 hologram;

FIG. 5 illustrates an embodiment working in the H1 master writing modefor the case that the holographic recording material is orientated atthe achromatic angle;

FIG. 6 illustrates an embodiment working in the H1 master writing modefor the case of a reflection H1 hologram;

FIG. 7 illustrates an embodiment working in the direct (1-step) writingmode for the case of a reflection hologram;

FIG. 8( a) illustrates the overlapping object beam density patternrecorded on the holographic material typical of an H1 master hologramwritten for the creation of a rainbow hologram by conventional transferwith each circle containing the perspective information for a certainviewpoint;

FIG. 8( b) illustrates the overlapping object beam density patternrecorded on the holographic material typical of an H1 master hologramwritten for the creation of a full-colour rainbow hologram byconventional transfer with each ellipse containing the perspectiveinformation for a certain viewpoint, the three rows representing thethree primary colour separations;

FIG. 9 illustrates the overlapping object beam density pattern recordedon the holographic material typical of an H1 full aperture masterhologram written for the creation of a mono or full colour reflectionhologram by conventional transfer with each circle containing theperspective information from a certain point in space as shown in FIG.1;

FIG. 10 illustrates the object beam density pattern recorded on theholographic material typical of a directly written hologram with eachcircle containing the directional and amplitude information of lightoriginating from that point that constitutes the 3-D image;

FIG. 11 shows an example of a wide angle objective used in oneparticularly preferred embodiment (optimized for 526.5 nm) having highresolution, low aberration, variable focal plane distance and a positionof minimum beam waist significantly outside the objective;

FIG. 12 shows a ray trace for the objective of FIG. 11 detailing variousplanes and key locations;

FIGS. 13( a)–(d) show spot diagrams for the objective of FIGS. 11 and 12calculated by reverse ray tracing from the object plane to the inputdata plane for four zoom configurations;

FIG. 14 shows ray intersection diagrams for the objective working atZoom 3 of FIGS. 11,12 and 13 at the object and input data planes;

FIG. 15 illustrates a known holographic printer given for illustrativepurposes only;

FIG. 16 illustrates an alternative known holographic printer; and

FIG. 17 compares a conventional method of producing a hologram with thecorresponding method of a preferred embodiment.

DETAILED DESCRIPTION OF THE INVENTION

5.1 Fundamental Image Data Required

In one embodiment of this invention a computer is used to generate athree dimensional model of an abject using a standard commercialcomputer program. Such computer programs can nowadays produce verylifelike models using a variety of sophisticated rendering processesthat mimic real life effects. In addition advances in computertechnology have now seen the calculation times, required for suchprograms to run, dramatically decreased. Three dimensional scannersusing Moire or other principles now permit the incorporation of realworld 3-D images in such computer models. The storage memory requiredfor such 3-D models is largely dependent on the texture maps usedtherein and hence computer files representing such 3-D models areusually relatively small and may be transmitted over the interneteasily. In the preferred embodiment of this invention said 3-D computermodels are used to generate a series of 2-D camera views from a virtualviewing plane as shown in FIG. 1. Here the viewing plane is labeled 101and individual 2-D images, such as 105 and 104, of the computerrepresented object 100 are generated at multiple locations on theviewing plane such as 102 and 103. The spacing and density of such 2-Dviews are generally controlled according to the information required fora certain type of hologram but in one embodiment they form a regular 2-Dmatrix and in another a regular horizontal 1-D array. Departures fromsuch regular forms are useful for various reasons such as, but notlimited to, reducing hologram image noise while controlling imageblurring.

In another embodiment of the invention a real model is used instead of acomputer representation and a real camera is employed to recordindividual photographs (either digitally or via photographic film thatis subsequently digitized). In such a case FIG. 1 should be interpretedin the following fashion. Object 100 represents the object to beholographed 101 represents the plane on which a camera 102 is positionedand photographs of the object 100 are taken at a variety of positions onthis plane. For example the view position 106 yields the photograph 105and the view position 103 yields the photograph 104. Generally somemechanism is used to transport a camera from position to position in asequential fashion using a 1 or 2 dimensional translation stage toaccomplish this. As before, the spacing and density of such 2-D viewsare generally controlled according to the information required for acertain type of hologram but in one embodiment they form a regular 2-Dmatrix and in another a regular horizontal 1-D array. Departures fromsuch regular forms are useful for various reasons such as, but notlimited to, reducing hologram image noise while controlling imageblurring.

In both of the above cases restricted animation, which may betransferred to the final hologram, may be modeled by arranging that themodel 100 moves in a defined sense (representing such animation) asdifferent camera positions are selected on the plane 101, such camerapositions following sequential monotonic trajectories on said plane. Onobserving the final hologram, an observer following such sequentialmonotonic trajectory in the observation space will perceive saidanimation.

5.2 Basic Principles

The preferred embodiment works by taking a set of 2-D views of a real orcomputer represented object and processing such views digitally togenerate data that is displayed on a spatial light modulator in twodimensions. According to a particularly preferred embodiment the spatiallight modulator is a high resolution liquid crystal display, but itshould be understood that, in less preferred embodiments, any other formof 2-D spatial light modulator having appropriate characteristics may beused.

In the preferred embodiment of the invention a pulsed laser is used toilluminate this spatial light modulator. Such pulsed laser may be asingle colour or a multiple colour laser and may produce pulses havingcharacteristic times from nanoseconds to tens of microseconds. Therepetition rate of such laser should ideally allow operation at speedsapproaching the refresh rate of the chosen spatial light modulator. Theuse of a pulsed laser allows the construction of a commercial machinethat is not affected by vibration. Hence high quality holograms may beproduced rapidly and predictably by the use of such device. The temporalcoherence and pulse to pulse energy variation of such laser should bechosen carefully. Generally, if object and reference beam arms areequalized the required temporal coherence is of the order of a fewcentimeters. The ultimate choice of pulse duration must depend of theindividual reciprocity relations of a given holographic recordingmaterial. If necessary pulse trains may be employed to achieve longerpulse envelopes whist preserving the peak electric field which is usefulfor non-linear frequency conversion.

A special illumination system for the spatial light modulator is usedthat controls the spatial coherence of the laser beam in an easilyadjustable fashion. In the preferred embodiment of the invention atelescope and micro-lens array are employed for this purpose although itmust be understood that other suitable systems for controlling, in aneasily adjustable fashion, the spatial coherence of a laser beam existand may be substituted. Such systems are characterized by those systemsthat control the spatial coherence of a laser beam in an easilyadjustable fashion whilst not introducing significant speckle noise. Theknown arrangement disclosed by Yamagushi et al. (“High Quality recordingof a full-parallax holographic stereogram with digital diffuser”,Optical Letters vol 19, no 2 pp 135–137 Jan. 20, 1994) uses apseudorandom diffuser directly in front of the SLM in order to limit thespatial coherence without inducing speckle noise. However, this systemdoes not allow the spatial coherence to be variably changed.

A microlens array consists of a two-dimensional ordered matrix ofmicro-lenses. Each lenslet has a certain diameter and focal length andthe array is characterized by the centre to centre spacing betweenadjacent lenslets. When illuminated by coherent light of high spatialcoherence each lenslet acts as an effective individual source andproduces a cone of diverging radiation. Downstream of the lens array,radiation from each lenslet will be superposed. A screen placed tointersect the plurality of radiation emanating from each lenslet will ingeneral show speckle noise. However, if the individual lenslets aresufficiently widely spaced then essentially there will be no speckle asthe phase information between individual sources becomes random. It is,however, important to understand that as the lenslet to lenslet distanceis increased the number of radiation sources in a certain area, A,decreases rapidly. The area, A, of the illuminated part of the lensarray essentially dictates, in the present system, the spatialcoherence. The number of radiation sources or lenslets within this areathen dictates the uniformity of the final SLM illumination beam throughensemble averaging. Since a pulsed laser is used in the presentinvention the techniques of beam cleaning that are used routinely in CWholography cannot normally be used due to electro-optic breakdown andthus the illumination beam is intrinsically less spatially uniform.Therefore as much ensemble averaging from the plurality of lensletsources as possible is required. In general we calculate optimum lensarray specifications and those of the lens array illumination telescopeby a combination of conventional raytracing and computationallycalculating the speckle pattern at the final hologram plane.

The laser light passing through the spatial light modulator passesthrough a special wide-angle objective lens that focuses the light intoa tight waist outside of such objective forming a beam known as theobject beam. An image of the spatial light modulator is formed at aspecific and controllable distance from the waist. An holographicrecording material is placed near or at such minimum waist of the objectbeam. A reference beam which is mutually coherent to this object beam isalso brought to illuminate the same physical region of the recordingmaterial but from a different angle such that the reference and objectbeams interfere in the region to produce an interference pattern whichis recorded by the recording material.

In one embodiment of the invention the holographic material is moved ina one or two dimensional fashion with respect to the object beam in aplane determined by the optimal overlap of the object and referencebeams, whilst the image on the spatial light modulator is changed suchthat each adjacent position of the object/reference beam pair on therecording material is encoded with an interference patterncharacteristic of such different computer data. Alternatively theobject/reference beam pair is moved and the recording material staysfixed (at least in one dimension). In either case such a method leads tothe creation of a plurality of individual interference patterns (knownhenceforth as holographic pixels) which form a 2-D matrix or one or more1-D arrays of such pixels. Such plurality of pixels is known as acomposite hologram.

It is desirable that the size and intensity distributions of both theobject and reference beams are accurately controlled dependent upon thetype of hologram being written and upon the required characteristics ofsuch hologram. In the case of the object beam this is done bycontrolling the spatial coherence of the laser light, in the case ofwriting a 1-step hologram, or by changing the distance of theholographic film from the wide angle objective in the case of an H1master hologram. The size of the reference beam may be effectivelycontrolled by image planing an aperture onto the recording materialsurface using an adjustable telescope, taking care to maintain beamcollimation and divergence within acceptable limits.

It is also desirable that a wide angle objective is designed andincorporated that both minimizes aberrations and which maintains alocation of minimum waist outside of such objective. The arrangementdisclosed by Yamagushi et al. (“Development of a prototype full-parallaxholoprinter”, Proc. Soc. Photo-Opt Instrum. Eng (SPIE) vol. 2406,Practical Holography IX, pp 50–56 February 1995) used a 3 lens objectivethat minimized spherical aberration (1^(st) Seidel coefficient) andachieved an f number of 0.79.

In general, the focal plane of the objective must be variable over asignificant range; in the case of an H1 hologram, the focused SLM imagedistance corresponds exactly to the H1–H2 transfer distance and hence tothe optimum viewing distance of the final H2 hologram. Aberrationscorresponding to higher order Seidel coefficients must be also beminimized. Accordingly, we have identified an appropriate class ofobjectives, an example of which is shown in FIG. 11 which haveexceedingly high field of view and the requisite properties of highresolution and low aberration over an extended range of focal planedistances. A characteristic of these objectives, which may be designedfor various laser wavelengths, is that they exhibit significant opticaldistortion (see FIG. 14 in which a perfect rectangle on the object plane1102 forms a rounded rectangle 1401 when traced back to the LCD which issituated on the input data plane 1101) and hence need to be correcteddigitally by software. Such “Pincushion” or “Barrel” type distortion maybe characterized in the canonical perturbation theory by a finite 5^(th)Seidel coefficient. F-numbers significantly smaller than those reportedin the prior art have been accomplished with the present objectives, andfields of view in the region of 100 degrees may be attained.

After writing, such composite holograms are processed according to theparticular requirements of the recording materials and a hologram iscreated. Preferred materials are Photopolymers and Silver Halides butother materials may also be used.

By suitable choice of the data processing algorithms many forms ofholograms can be generated by the above process.

Two important classes of hologram may be distinguished. The first areholograms known as H1 holograms which are designed to be transferred toanother hologram (henceforth referred to as the H2) in which the planeof the 3-D image is changed. Such image-plane transferring has beendescribed above and is a standard classical optical technique. Thesecond class of hologram is a hologram that mimics directly thistransferred or H2 hologram avoiding the requirement to pass through theH1 stage. In this case the plane of the 3-D image is changed by using acomputer to perform a different mathematical manipulation algorithm onthe original data set.

As a person skilled in the art will appreciate these two differentclasses of hologram require significantly different writing conditionssuch that optimum hologram quality be attained for both classes. H1holograms are best written with large pixels which may reach an area onthe recording material of hundreds of times larger than the pixelsrequired to write directly the final hologram. Each pixel is thusoverlapped many times. This results in an H1 hologram that is reduced inbrightness but which is fundamentally less noisy. The technique of imageplane transferring is then able to compensate for this reduction indiffraction efficiency and the result is an H2 hologram that isoptimally bright and of a very high quality.

Directly written holograms require an abutting pixel structure which isminimally overlapped if final hologram brightness is not to becompromised. This, of course, places constraints on the final imagequality for certain applications.

5.3 Description of Preferred Embodiments

The following describe the preferred embodiments of the presentinvention which serves to describe and illustrate the principles of suchinvention. However it should be clear that those skilled in the art canmake various modifications, additions and subtractions without departingfrom the scope of the invention. For example, an optical system may bearranged in a multitude of ways. The system for the advance and movementof the recording material relative to the spatial light modulator may beconstructed also in numerous ways and rigid substrates instead of theflexible material used below may be employed.

5.3.1 The Object Beam Arm

FIG. 2 shows an overhead view of the preferred embodiment of theinvention. A single colour single-frequency pulsed laser 200 (Nd:YLFsingle-oscillator flash-pumped second harmonic (526.5 nm) singlefrequency laser giving 1 ml per pulse in one embodiment) capable ofrapid operation and having sufficient temporal coherence emits a beam ofcoherent light which is split by a variable beamsplitter 201. The beam202 continues to the mirror 203 whereupon it is diverted to the mirror204 whereupon it is diverted to the waveplate 205 which controls thepolarization of the beam. The beam continues to a telescope comprisinglenses 206, 207 and 265. Lens 207 is mounted on a motorized translationstage 208 with motor 209. The diameter of the beam exiting from optic207 is thus controlled and approximately collimated. The beam passes tothe micro-lens array 210 which expands the beam onto the collimatinglens assembly 211. The distance between the elements 210 and 211 ischosen to be the effective focal length of the lens 211. In such a way a“collimated” beam exits the optic 211 with a controllable spatialcoherence. The beam now illuminates a liquid crystal display (LCD) 212,having resolution 768×1024 pixels and lateral dimension of 26.4 mm,which is mounted on a 2-D motorized translation stage 216 havingvertical control motor 215 and horizontal control motor 218. Positionsof maximum LCD horizontal displacement are indicated by 213 and 214. TheLCD position is adjusted when writing H1 type holograms and is used toattain a much higher resolution of final image than would otherwise bepossible with the same static LCD for a given angle of view. The LCDposition may also be adjusted when writing a 1-step hologram in order tomaintain a particular hologram viewing window geometry.

After passing through the liquid crystal display, the beam traverses alinear polarizer that converts the LCD image from a polarizationrotation image into amplitude modulation. Then the beam passes throughthe wide-angle objective 219 mounted on the motorized translation stage220 with motor 263. This stage is used to control the position of thefocused image (1102 in FIG. 11) of the LCD produced by the objective219. The size of the minimum waist 266 of the object beam is controlledby the motorized stage 208 with motor 209. The object beam now comes tobear on the hologram material 262 here shown as film mounted on aroll/stage system. The motor 229 controls movement of the stage 223towards and away from the position of minimum object beam waist. Therollers 224 and 225 control the horizontal movement of the film 262 infront of the object beam. The motor 228 controls the vertical movementof the film in front of said object beam. Motor 226 controls the motionof the rollers 224 and 225. Rollers 222 and 231 tension the film andcontrol the horizontal angle that the film makes to the axialpropagation vector of the object beam. For example FIG. 5 shows asection of this diagram for the case that the film is pulled back to theachromatic angle which is useful when writing H1 masters for transfer topanchromatic rainbow H2 holograms.

5.3.2 The Reference Beam Arm

The reference beam is split from the main laser beam by the variablebeamsplitter 201 controlled by motor 265. The beam 235 is directed to amirror 236 whereupon it is reflected through an quasi-elliptical orrectangular aperture 237, an effective image of which is eventuallycreated at the intersection of the reference beam with the holographicrecording material, such quasi-elliptical or rectangular shape producinga defined circular or quasi-elliptical or rectangular referencefootprint on the recording material as may be required by the type ofhologram being written. The reference beam continues to the waveplate238 which controls the polarization of the laser beam. The elements 239and 241 with either 264 or 263 form a telescope that controls the sizeof the beam after 264/263 which is adjustable by the motorized stage 242with motor 243. The beamsplitter switch 244 either directs the referencebeam on the path 254 or onto the path 245. Path 245 is used to createtransmission holograms whereas path 254 is used to create reflectionholograms.

In the case of path 245 the reference beam passes through the lens 264that produces an approximate image of the aperture 237 at the recordingmaterial surface. This lens also corrects for the slight divergence ofthe light produced by the lens 241. The divergence of the light after264, which is ideally collimated, is thus controlled to withindiffraction limits. Practically this means that for small reference beamsize the beam will not be exactly collimated but that such departurefrom collimation will lead to an image blurring significantly less thanthat induced by the source size of the final hologram illuminationsource. Mirrors 246 and 249 now direct the reference beam onto itstarget to intersect the object beam at the surface of the holographicrecording material. Motorized rotation stages 247 and 250 with motors248 and 252 respectively and the linear translation stage 251 with motor253 assure that different reference angles may be achieved for differentplacements and orientations of the recording material. For manyapplications Brewster's angle is to be preferred but some applicationsspecifically require the flexibility to change this angle.

In the case of path 254 the reference beam passes through the lens 263that produces an approximate image of the aperture 237 at the recordingmaterial surface. This lens also corrects for the slight divergence ofthe light produced by the lens 241. The divergence of the light after263, which is ideally collimated, is thus controlled to withindiffraction limits as above. Mirrors 255 and 256 now direct thereference beam onto its target to intersect the object beam at thesurface of the holographic recording material, this time from theopposite side to the object beam. The motorized rotation stage withmotor 259 and the linear translation stage 258 with motor 260 assurethat different reference angles may be achieved for different placementsand orientations of the recording material. For many applicationsBrewster's angle is to be preferred but some applications specificallyrequire the flexibility to change this angle.

FIG. 3 shows a perspective view of selected components of the preferredembodiment numbered to correspond to FIG. 2.

5.3.3 H1 Transmission Holograms

By far the most frequently encountered type of H1 hologram is the H1transmission hologram. This type of hologram comes in four basicvarieties (i) H1s suitable for making rainbow transmission holograms;(ii) H1s suitable for making panchromatic (i.e. full colour) rainbowtransmission holograms; (iii) H1s suitable for making achromatic (i.e.black and white) transmission holograms; and (iv) H1s suitable formaking single colour reflection holograms. In all cases the individualholographic pixels should be well overlapped and much larger than theminimum waist size of the object beam in order to distribute theinformation of a particular perspective over a macroscopic area of thehologram and to insure good averaging of such spatial optical noise thatis inherent in the system.

FIG. 4 shows a diagram of the system in H1 transmission mode. Note thatthe reference beam comes in towards the recording material from the sameside as the object beam to form a pixel 221. Note that said pixel issignificantly displaced from the point of minimum waist 266. Note thatthe image (at the plane 1102 in FIG. 1) of the LCD 212 is located at adistance 401 from the recording material 262 and that a screen placed at402 would show a sharply focused image of each 2-D picture loaded intothe LCD 212. The plane 402 (1102 in FIG. 11) usually corresponds to theH2 plane in a transfer geometry.

In order to record an H1 transmission hologram perspective views of areal or computer generated object are pre-distorted to compensate forresidual optical distortion and for a certain final lighting geometry.Such images are then loaded into the LCD one by one, a holographic pixelrecorded, the recording material advanced and the process repeated foreach image. For the case (i) above a line of pixels is written on theholographic recording material as illustrated in FIG. 8( a). Each circlerepresents an interference pattern containing information about acertain perspective view along a horizontal viewing line (note that inreal life the individual pixel shape is not exactly circular but we haveused this shape as a clear representation for the purposes ofillustration). FIG. 8( b) illustrates the case (ii) where three lines ofpixels are written at the achromatic angle each line corresponding to ared, green or blue component image in the axial viewing position of thefinal hologram. The recording geometry for case (ii) is shown in FIG. 5.FIG. 9 shows cases (iii) and (iv) where a 2-D array of pixels must bewritten. In the case of (iii) all the horizontal lines of pixelsactually contain information relating to a single vertical parallax. Incase (iv) this may or may not be the case. However, if full parallax isused the packing density of the pixels may be modulated to reducechromatic blurring of the image. In fact the packing density maygenerally be modulated to optimize the reduction of optical noise byensemble averaging whilst the clarity of an individual image ismaintained from image blurring by a close partner pixel. Generally theseconsiderations are more important for large reflection type fullparallax holograms which suffer from chromatic blurring in the limitthat infinitely many views are used to construct the stereogram.Nevertheless very large reduced parallax holograms should be optimizedif blurring and noise are to be held in check.

In all cases the spatial coherence of the object beam must be controlledsuch that the size of the minimum waist in the object beam subsequent tothe objective be controlled. This minimum waist determines once again,the freedom from blurring of the image whilst improving the imagequality. Hence too small a waist and the image quality will be bad andtoo large a waist and the image will be blurred. There is however a verylarge range of waist sizes in between extremes of these two parametersand it is highly desirable to accurately choose an optimum waistdiameter. This is why it is a particularly preferred feature of thepresent invention to employ a method of controlling the spatialcoherence of the object beam that easily permits said coherence to bechanged.

The optimum maximum holopixel packing density of an H1 transmissionhologram should be determined ultimately by the type of recordingmaterial used. In certain applications such as full-colour holography, areflection H1 hologram is to be preferred over a transmission H1hologram. In such a case the single frequency colour laser is replacedby a multi-colour single frequency laser and the LCD may, for example bereplaced by a colour LCD or other spatial light modulator. In this casea colour H1 master hologram can be written with the geometry shown inFIG. 6 and that may be transferred by image planing to a colourreflection H2 hologram. The holopixel packing density of such H1reflection holograms may well be slightly different to the pixel packingdensity preferred on H1 transmission holograms and this will depend onthe characteristics of a given recording material.

5.3.4 Directly Written Holograms

When a hologram is directly written (1-step process) one can no longercontrol the final hologram brightness through an image planing transferprocess. Hence the hologram that is being written should be of optimalbrightness. This means that the holographic pixels must be abuttedrather than overlapped as illustrated in FIG. 10. Consequently theoptimum position for the holographic film is at the position of minimumobject beam waist as illustrate in FIG. 7. The system of object beamspatial coherence control already described is now used to control thesize of the holographic pixel and to assure that its intensitydistribution on the recording material surface is approximatelygaussian.

In order to record a directly written hologram, perspective views of areal or computer generated object are mathematically transformed tocreate a set of new images which are then pre-distorted to compensatefor residual optical aberration and for a certain final lightinggeometry. Such images are then loaded into the LCD, a holographic pixelrecorded with the image plane of the LCD being set optimally to theintended viewing plane or to infinity and then the recording materialadvanced and the process repeated. The process is carried out in such away as to create a two dimensional matrix of holographic pixels, witheach such pixel reproducing faithfully the rays of light intersecting acorresponding point on a chosen image plane passing through the real orvirtual computer object. Under certain approximations this techniquethus produces a hologram identical to a hologram that is produced as aH1 master hologram and then transferred using classical image planing tomake an H2 hologram. However, in practice there are large differencesand the two techniques are rather complimentary and, as discussed above,have preferred uses for different applications.

Different mathematical transforms may be constructed that create all themain types of holograms using the technique of directly writing thehologram. Rainbow holograms may be constructed by arranging that theindividual LCD image files for a given holographic pixel consist of asingle horizontal band of information. The height of this band on theLCD is chosen to depend on the vertical position of the holographicpixel. In this way a hologram is created that focuses its illuminationlight into a horizontal band in front of the hologram. This band ismodulated with the image information from a single vertical perspectivethus creating a rainbow hologram. In the case of a three colour rainbowholograms the image files for each holographic pixel consist of threehorizontal bands whose vertical positions in the LCD depend differentlyon the vertical position of the respective holographic pixel.Consequently the final hologram acts to focus its illumination lightinto three horizontal bands parallel and in front of the hologram, thistime the three bands lying on a plane orientated at the achromatic angleto the hologram's normal vector. Again each band is modulated with therespective primary colour image information from a single verticalperspective thus creating a panchromatic rainbow hologram. Amonochromatic single parallax reflection hologram is created by LCDimage files composed of vertical stripes horizontally modulated with thehorizontal perspective information. Full parallax reflection hologramsare likewise created by truly 2-D transformed LCD image files.

By playing with the mathematical transforms one is able to generatehybrid holograms by the direct writing technique such that the imageappears achromatic from one perspective but perhaps has a rainbowcharacter from another viewpoint. Alternatively many different viewingwindows can be constructed for the holograms with ease and parametersincluding the intrinsic image blurring may be controlled to produce verylarge depth views from certain angles whilst other angles may beoptimized for image integrity of nearer objects.

5.3.5 Other Techniques

The wide angle objective used in the embodiment described here above isillustrated and defined in FIGS. 11 to 14. This 85° objective has beendesigned for operation within a range of focal distances between 50 cmand 1.5 m. FIG. 13 shows standard spot diagrams for 4 zooms in betweenthese extremes (zoom 1 corresponds to a magnification of 45×, zoom 2 to31.8×, zoom 3 to 22.5 and zoom 4 to 15.9×). As can be seen the maximumprojected spot size on the input data plane is not more than the pixelsize of the LCD (approx 50 microns). The optical distortion of thisparticular objective is around 6%. By accepting a slightly higher valueof 12% later versions have in fact improved resolution to well under thepixel size of the LCD.

The objective is designed to work with an LCD having lateral size 26.4mm. However the input data plane of the objective (1101) is 61.7 cmwide, thus permitting significant lateral and up/down movement of theLCD within said plane. By insuring that our objective works within itsdesign limits between a focal distance of 50 cm, where it gives amagnification of 15.9× to 1.5 m where it gives a magnification of 45× weare able to create maximum resolution 2-step holograms of sizes from30×40 cm to larger than 1 m×1 m, all having appropriate optimum viewingdistances. In connection to this latter point, one should note that theH1–H2 transfer distance is chosen to be equal to the objective focaldistance used which is also then the optimum viewing distance of thefinal H2 hologram. (In the case of 1-step holograms there is noeffective limit on hologram size supposing that one can arrange for acollimated replay illumination).

Frequently techniques for controlling the spatial coherence of theobject beam lead to the introduction of noise into this beam. The mostcommon is laser speckle and will be excluded from further considerationsince the preferred embodiment controls the spatial coherence withoutintroducing significant speckle. In the preferred embodiment of thisinvention a telescope and a micro-lens array has been used to achievesuch control. However the physical construction of micro-lenses is proneto introduce some optical pattern into the object beam. This noise canbe significantly reduced by moving the micro-lens array at each exposurein a random or ordered fashion. Such noise is also greatly reduced bychoosing the optimum maximum pixel density as described above.

When writing H1 holograms the SLM can be moved both horizontally andvertically within the input data plane (1101) of the objective forsequential writing operations. This effectively allows the use of asmaller and lower-resolution SLM than we would otherwise have had to useif software image control were exclusively relied upon in order toachieve the same angle of view and resolution in the final H2 hologram.In the case of an H1 hologram for production of a rainbow hologram theSLM is only moved in one direction. However, for H1s for 3-colourrainbow holograms. H1s for reflection holograms or full parallax H1s theSLM must be moved in a 2-D sense.

The SLM may also be moved both horizontally and vertically within theinput data plane (1101) of the objective when 1-step holograms are beingwritten. In this case it is possible to modify advantageously the finalviewing window of the hologram. Specifically we are able to make ahologram where said hologram is either completely viewable from acertain viewing zone or completely invisible. This should be contrastedwith the case of a static SLM where optimum use of the SLM dictates thatlarge portions of the hologram viewing zone show only a partial view ofthe hologram image.

In practice the hybrid technique of both using software control and somemovement of the SLM in the objective input data plane (1101) can also beemployed for both 1-step and H1 hologram generation.

Alternatively, where a very high resolution SLM is available, a largerstatic SLM effectively filling the input data plane (1101) of theobjective will provide a better solution, all image manipulation nowbeing done exclusively by software.

In the case of writing an H1 hologram the holographic film should beheld at some significant distance from the location of the minimum beamwaist (266). Since that part of the objective input data plane (i.e.containing the LCD) that is transmissive to laser light is always muchsmaller than the entire objective input data plane (1101) and furtherthat this transmissive area must move from shot to shot due either tosoftware image control or to the fact that the SLM is physically movedin the input data plane (at least one of which options is criticallyrequired for the invention to work correctly), it is to be noted thatthe zone of object radiation falling on the holographic film 262 at 221also inevitably moves from exposure to exposure. The embodiment of FIG.2 may therefore be further improved by arranging to move in a twodimensional automatic fashion the aperture 237 such that the referencebeam shape, size and position on the holographic film plane effectivelymatches the object beam shape, size and position thereon. Thisimprovement is not required when 1-step holograms are written as theposition of the holographic film effectively coincides with the minimumwaist point of the object beam. In this case the size, position andshape of reference and objects beams at the film plane are matched onetime before for all exposures begin, rather than constantly tailoringthe reference beam- and thus a moveable aperture is not usuallyrequired. Note that in FIG. 2 an aperture 237 was used to definereference beam shape and an adjustable telescope 239, 241, 263, 264 todefine beam diameter. Further controllable mirrors (246 and 249 forexample) then change the reference angle to the film plane 262.Generally these individual systems cross-link and software control mustwork out how to match the reference and object beam size, position andshape at the film plane in the best fashion. Clearly non-matched objectand reference beams at the film plane will lead to image quality andbrightness reduction.

In the embodiment of FIG. 2 the capability of changing the referenceangle is to be noted. This is useful for a variety of reasons such ascompensation for emulsion swelling on chemical processing, for thegeneration of H1 holograms designed for transfer at other laserwavelengths, for writing rainbow masters on a single (achromatically)tilted substrate and for creating holograms that are to be illuminatedby a diverging or converging white light beam. One might note, however,that the aspect ratio of the aperture 237 must be changed as thereference angle is changed in order that object and reference footprintscan properly match on the holographic film. In fact in order to attaincomplete matching of object and reference footprints one must arrangefor a variably controllable variably magnified aperture. Thiscomplication is not evident in FIG. 2. In the case that a holographicprinter must be able to print not only 1-step holograms having a pixelsize of less than 1 mm but also H1 type holograms having pixel sizes ofseveral cms, significant care must be exercised in the design of thereference beam preparation system. In this case elements 237, 239, 241,263 and 264 may be individually complex elements, an automatic systemfor controlling the aperture size and aspect ratio may be present andsoftware may link the system back to both the laser energy output andthe object/reference ratio.

In many cases, 1-step holograms are illuminated for display with anon-collimated beam of white light emanating from a point source such asa halogen lamp. If account is not taken of the replay illuminationgeometry and further a constant angle of reference is employed atrecording, both image distortion and viewing window distortion willoccur on illumination of the 1-step hologram by a diverging beam. Byusing a combination of image pre-distortion based on a diffractivemodel, one or two-dimensional translation of the LCD and by moving thereference beam in only one dimension at each holopixel exposure, anyinduced image distortion can be compensated for and a very much improvedhologram viewing window may be attained. Thus the combination of aone-dimensionally changeable reference beam, a translatable LCD andsoftware image distortion are highly desirable, particularly for largerholograms. Usually the reference beam is required only to be changedover a relatively minor range of angles in order to compensate for anon-collimated replay beam and hence the above complication of anautomatic system for controlling the reference beam control aperturesize and aspect ratio is desirable only in certain cases such as, forexample, when smaller 1-step holograms are tiled together to form largerpanels. By the use of a static LCD it is possible to produce hologramsthat are designed for point source illumination but higher resolutionSLM panels are then required. Since commercial SLM resolution is limitedand it is desired to produce an optimum hologram quality it is hencedesired to be able to translate said SLM. In principle a two-dimensionalangular manipulation of the reference during recording is possible butin practice we find that the added mechanical complication of such atwo-dimensional steering system is not merited and practically suchsystem provides no substantial advantage.

During normal operation of the preferred embodiment severalelectromechanical precision stages may be required to update theirposition at exposure. When the laser (200) is operated at above a few Hzcertain electromechanical problems need thus to be addressed as itbecomes no longer possible to stop and start said electromechanicalstages without the induction of unacceptable mechanical vibration with,for example, the associated loss of positioning precision.

This problem has been addressed by constructing a microprocessor-basedcontroller which is capable of setting up different constant andnon-linear programmed velocity trajectories on multiple stages. It iscurrently possible to run this system up to 30 Hz with excellentmechanical vibration characteristics.

In FIG. 5 a method for writing an H1 hologram suitable for transfer to afull colour rainbow hologram is shown. The film is shown pulled back bythe roller 231 into the achromatic position. It should be noted,however, that this constitutes only one such way by which H1s suitablefor this application may be written. In particular it may be decided towrite the 3 or more strip master holograms (see FIG. 8 b) with the filmin the flat position as indicated in FIG. 2 rather than in the positionindicated in FIG. 5. In this case the software and image focusing stageare adjusted to change key properties of the interference patternwritten for each strip. A particular image plane transfer system is thenused whereby the 3 strips are separated and aligned in a staggeredgeometry at the achromatic angle but individually all parallel to thefinal H2. Such a method has practical advantages concerning imagequality, precision alignment and machine calibration over the simplermethod covered above and that is illustrated in FIGS. 5 and 8 b.

It should be noted that the pulsed laser 200 in the above embodiment hashigh temporal coherence and thus there is no prevision for adjusting theobject and reference pathlengths therein. However, if a pulsed laser isemployed which has a lesser temporal coherence then in accordance with aless preferred embodiment the object and reference paths are equalizedand in the case that such coherence is marginal such equalization may beelectromechanically controllable.

By arranging, for the case of a holographic printer employing a wideangle objective of very low aberration, that the vertical and horizontalmovements of both the holographic material and the spatial lightmodulator are synchronized such that the image at the final H2 plane ofthe pixels of the spatial light modulator line up for all the 2-D imagesprojected in the creation of an H1 hologram in such a manor that such H1hologram, when transferred to an H2 hologram, will create a definedpixelated image on the surface of said H2 hologram, then the images onthe spatial light modulator can be decomposed and encoded intointerlaced groups of pixels representing several primary colours and aregistered coloured mask may be attached, laminated or printed onto saidH2 hologram to produce a multiple colour hologram.

5.4 Modifications to the Preferred Embodiment

As mentioned above, the preferred embodiment may employ a monochromaticpulsed laser or a multiple colour pulsed laser. The principle advantageof using a multiple colour laser is that multiple colour or full colourreflection holograms may be printed either using the 1-step or by usingthe 2-step method.

There are several choices on how to implement a multiple-colour pulsedlaser in the preferred embodiment. Referring to FIG. 2A the first is tosimply construct several separate and distinct optical systems of thekind described above and shown in FIG. 2 one for each colour produced bythe laser. This way, if it is assumed that said multiple colour laserhas three emission wavelengths, essentially three of everything isrequired including three sources 200 a, 200 b, 200 c, three controllablelens telescopes 207 a, 207 b, 207 c, three lens systems 219 a, 219 b,219 c, three SLMs 212 a, 212 b, 212 c, and three objectives 202 a, 202b, 202 c and three reference 235 a, 235 b, 235 c beams. It is thereforepossible to write holographic pixels three times as fast, but of coursethere are also three times as many pixels to write. If this method ofimplementation of a multiple colour laser is chosen the film (or plate)advance system is organized such that it supports three separateconcurrent write locations. In addition the spacing between thedifferent colour holographic pixels is controlled such that in one caseit is arranged that pixels of differing colour line up and in anothercase a well-defined chromatic pixel juxtaposition is created.

The other way this problem has been tackled is by creating an opticalsystem with a changeable writing objective that otherwise functionsequally well for each of the component wavelengths. It is effectivelyimpractical to create an optical system in its entirety that functionsfor many wavelengths at the same time. This is primarily due to the veryspecialized objective that we must use which depends on its design foruse at only one wavelength. Hence this problem is solved by exposingfirst with one colour, then with another and so-forth. Each time thecolour is changed the the appropriate writing objective iselectromechanically selected. With three colours three objectives in aprecision mount are used, each of which can be loaded at call.

In summary the present invention provides a method and apparatus forwriting all the major types of 1-step and intermediate H1 type hologramsof high quality, at speeds fundamentally faster than the prior-art andwithout practical constraints of operation on vibration.

1. A holographic printer for directly writing 1-step white-lightviewable holograms, comprising: a pulsed laser source arranged toproduce a laser beam at a first wavelength, said laser beam being splitinto an object beam and a reference beam which is mutually coherent withsaid object beam; a spatial light modulator wherein, in use, said objectbeam illuminates said spatial light modulator; a lens system for writinga holographic pixel of a hologram on to a photosensitive medium, saidlens system being arranged downstream of said spatial light modulator;positioning means for positioning a photosensitive medium at the Fourierplane of said lens system; and automatic spatial coherence varying meansfor automatically varying the spatial coherence of said object beam soas to control the diameter of said object beam at said Fourier plane. 2.A holographic printer as claimed in claim 1, wherein said automaticspatial coherence varying means is arranged upstream of said spatiallight modulator.
 3. A holographic printer as claimed in claim 1, whereinsaid spatial light modulator is translatable.
 4. A holographic printeras claimed in claim 1, wherein said automatic spatial coherence varyingmeans comprises an adjustable telescope and a microlens array.
 5. Aholographic printer as claimed in claim 4, wherein said adjustabletelescope is arranged to create an approximately collimated variablediameter laser beam that illuminates said microlens array.
 6. Aholographic printer as claimed in claim 5, wherein said adjustabletelescope may be adjusted so as to vary the diameter of the object beamat the Fourier plane.
 7. A holographic printer as claimed in claim 1,further comprising means for modifying images sent to said spatial lightmodulator so as to at least partially correct for inherent opticaldistortions of said lens system.
 8. A holographic printer as claimed inclaim 1, wherein said lens system has an effective field of viewselected from the group consisting of: (I) greater than 70 degrees; (ii)greater than 75 degrees; (iii) greater than 80 degrees; and (iv) atleast 85 degrees.
 9. A holographic printer as claimed in claim 1,wherein the Fourier plane of said lens system is located downstream ofsaid lens system.
 10. A holographic printer as claimed in claim 9,wherein the Fourier plane of said lens system is located at least 2 mmdownstream of said lens system.
 11. A holographic printer as claimed inclaim 1, wherein said laser source is arranged to additionally producelaser beams at second and third wavelengths.
 12. A holographic printeras claimed in claim 11, wherein said first, second and third wavelengthseach differ from one another by at least 30 nm.
 13. A holographicprinter as claimed in claim 1, further comprising a second pulsed lasersource for producing a laser beam at a second wavelength.
 14. Aholographic printer as claimed in claim 13, further comprising a secondlens system for use at said second wavelength.
 15. A holographic printeras claimed in claim 14, further comprising a third pulsed laser sourcefor producing a laser beam at a third wavelength.
 16. A holographicprinter as claimed in claim 15, further comprising a third lens systemfor use at said third wavelength.
 17. A holographic printer as claimedin claim 16, wherein said first, second and third wavelengths eachdiffer from one another by at least 30 nm.
 18. A method of directlywriting 1-step white-light viewable holograms, comprising: providing apulsed laser source arranged to produce a laser beam at a firstwavelength; splitting said laser beam into an object beam and areference beam which is mutually coherent with said object beam;illuminating a spatial light modulator with said object beam; providinga lens system for writing a holographic pixel of a hologram on to aphotosensitive medium, said lens system being arranged downstream ofsaid spatial light modulator; positioning a photosensitive medium at theFourier plane of said lens system; and automatically varying the spatialcoherence of said object beam so as to control the diameter of saidobject beam at said Fourier plane.
 19. A holographic printer fordirectly writing 1-step white-light viewable holograms, comprising: apulsed laser source arranged to produce a laser beam at a firstwavelength, said laser beam being split into an object beam and areference beam which is mutually coherent with said object beam; aspatial light modulator wherein, in use, said object beam illuminatessaid spatial light modulator; a lens system for writing a holographicpixel of a hologram on to a photosensitive medium, said lens systembeing arranged downstream of said spatial light modulator, wherein saidlens system has an effective field of view selected from the groupconsisting of: (I) greater than 70 degrees; (ii) greater than 75degrees; (iii) greater than 80 degrees; and (iv) at least 85 degrees;means for modifying images sent to said spatial light modulator so as toat least partially correct for inherent optical distortions of said lenssystem; and positioning means for positioning a photosensitive medium atthe Fourier plane of said lens system.
 20. A method of directly writing1-step white-light viewable holograms, comprising: providing a pulsedlaser source arranged to produce a laser beam at a first wavelength;splitting said laser beam into an object beam and a reference beam whichis mutually coherent with said object beam; illuminating a spatial lightmodulator with said object beam; providing a lens system for writing aholographic pixel of a hologram on to a photosensitive medium, said lenssystem being arranged downstream of said spatial light modulator,wherein said lens system has an effective field of view selected fromthe group consisting of: (I) greater than 70 degrees; (ii) greater than75 degrees; (iii) greater than 80 degrees; and (iv) at least 85 degrees;modifying images sent to said spatial light modulator so as to at leastpartially correct for inherent optical distortions of said lens system;and positioning a photosensitive medium at the Fourier plane of saidlens system.